The present invention relates to a conductive polymer capacitor and method for making same. Prior solid electrolytic capacitors have been provided from a molded porous body of metal such as tantalum, niobium, or aluminum which serves as an anode for the capacitor. The metal is powdered and is formed by heat and pressure into a solid porous body. An oxide coating is formed over the metal and a solid electrolyte such as maganese dioxide (MnO2) is formed over the dielectric and serves as the cathode of the capacitor.
In recent years polymers have been used in forming the cathode solid electrolyte. Examples of the use of polymers as a solid cathode electrolyte are shown in U.S. Pat. Nos. 5,461,537; 4,780,796; and 5,457,862.
In these prior capacitors, two polymer layers are formed. The first polymer is formed by using a chemical oxidant to chemically oxidize and polymerize the polymer on the dielectric layer of the capacitor. Then a second polymer layer is formed using a solution containing an undoped polymer compound polymerized in advance and soluble in an organic solvent. After the formation of the second polymer compound, the second polymer compound is doped to make the undoped polymer compound layer conductive.
Therefore, a primary object of the present invention is the provision of an improved conductive polymer capacitor and method for making same.
A further object of the present invention is the provision of an improved conductive polymer capacitor which achieves very high conductivity of the polymer layers, thereby resulting in a capacitor with lower equivalent series resistance (ESR).
A further object of the present invention is the provision of an improved conductive polymer capacitor and method for making same which is more durable in use, more reliable in operation, and more efficient to manufacture.
The foregoing objects may be achieved by a conductive polymer capacitor having an anode formed from a porous metal body, and including an anode lead extending therefrom. A dielectric layer is formed on the surface of the anode by oxidizing the metal of the anode. A solid electrolyte is formed on the dielectric layer comprised of a first conductive polymer layer formed on the dielectric layer and a second conductive polymer layer formed on the first conductive polymer layer. The first conductive polymer layer is preferably a polypyrrole layer. The second conductive polymer layer comprises a polyaniline layer formed by dipping the metal body having the first conductive polymer layer thereon into a solution of doped polyaniline dissolved in an organic solvent.
As used herein the term xe2x80x9cdopedxe2x80x9d refers to the use of a dopant which is either an electron donor or acceptor in polymer compounds such as polypyrrole, polythiophene, and polyaniline. Numerous types of dopants are well known in the art of making capacitors, and include, but are not limited to, naphtalenesulfonic acid sodium salt, toluenesulfonic acid sodium salt, and benzenesulfonic acid sodium salt. Other dopants capable of donating or accepting electrons to the polymer compound may be used without detracting from the invention.
Numerous organic solvents may be used for the dissolving of the polyaniline before the dipping process. Examples include but are not limited to m-Cresol, p-Cresol, o-Cresol, and Cl-Cresol.
In one form of the invention inorganic fillers may be included within the polyaniline solution inorganic solvent so that the second conductive layer, once formed by dipping, includes the inorganic filler within the second conductive layer. The inorganic filler may be selected from any of a number of inorganic fillers, including but not limited to graphite, SiO2, Al2O3, SrO2, ZrO2, MgO, BeO.